This invention relates to polyamides having a particularly advantageous combination of chemical and physical properties. More particularly, this invention relates to polyamides which are particularly suited for use as hot melt adhesives.
Film-forming or fiber-forming polyamide resins such as nylon have been the subject of extensive research since the 1930's. Much of this research has been directed toward improving the mechanical and rheological properties, and processability of polyamide resins. Recent research has involved the evaluation of the physical properties of polyamides resulting from the copolymerization with other polyamide forming species of newly developed dimeric, long chain, dibasic fatty acids (commonly referred to as `dimer acids`) or dimer diamines which are dimer acids which have been converted to amines. Generally the direction of the aforementioned polyamide research has been to polymerize substantial amounts of dimer acids (or their amine derivatives) with other polyamide-forming species to improve the mechanical and rheological properties and the processability of the resulting polyamides. For example, U.S. Pat. No. 4,062,820, British Pat. No. 1,319,807, and U.S. Pat. No. 3,549,573 disclose the condensation polymerization of 31 to 51 mole percent (based upon total moles present) of a dimer acid with the remainder of the resulting polyamide comprised of conventional short chain dibasic organic acids and short chain organic diamines (hereafter "short chain polyamide-forming species") and various other species which improve the properties of the polyamides. The polyamides of the previously mentioned patents exhibit undesirable impact resistance or would not be suitable for use as hot melt adhesives due to excessive melt viscosities.
A patent which described polyamides having a limited amount of dimer acids (the remainder comprised of short chain polyamide-forming and other property improving species) is U.S. Pat. No. 4,018,731. Such polyamides which have been modified with very small amounts of dimer acid (e.g., 0.5 mole percent), when prepared as low melt viscosity resins, are excessively crystaline and brittle, exhibiting low impact strength.
The present invention provides polyamides with unexpectedly advantageous properties (particularly impact strength) by polymerization of intermediate amounts of dimer acids or dimer diamines (collectively designated hereafter as polyamide-forming dimer acids) with long chain polyamide-forming oligomers, prepolymers, or other polyamide-forming polymers and with mixtures of conventional short chain polyamide-forming species. The condensation polymerization of materials chosen from the above three classes produces polyamides which exhibit useful adhesive properties, are tough, flexible, and impact resistant and are advantageously suited for use as hot melt adhesives. The present polyamides also exhibit little or no phase separation at room temperature or elevated temperatures. Additionally, the softening point of the present polyamides is controllable by the choice of the short chain polyamide-forming components.
More particularly, the present invention provides, in one aspect, a copolyamide having a melt viscosity of less than 100 poise at 232.degree. C., an impact resistance of at least 180 kg-m/square meter at -180.degree. C. and at least 890 kg-m/square meter at 21.degree. C. and a ball and ring softening temperature of at least 110.degree. C., comprising the condensation product of
a. 57.5 to 94.75 mole percent short chain polyamide-forming species or moieties; PA1 b. 5 to 30 mole percent polyamide-forming dimer acid species or moieties; PA1 c. 0.25 to 12.5 mole percent amorphous polyamideforming oligomer, pre-polymer, or polyamideforming polymer species or moieties having a number average molecular weight in the range of about 600 to 10,000, preferably 900 to 5,000.
In another aspect, the present invention provides a hot melt adhesive with particularly desirable properties.
It should be noted that all three of the generic classes of compounds which comprise the present polyamides are "polyamide-forming". "Polyamide-forming" herein means materials or compounds having functionality directly or indirectly capable of entering into a reaction which produces a polyamide, i.e. a polymer characterized as having within the structure at least some polyamide functionality of the formula, ##STR1## the functionality being connected with any of the various linking segments which may be used to connect the above described polyamide functionality. This definition is specifically meant to include species which form a polyamide while undergoing such reactions as ring opening, e.g., cyclic lactams; desterification, e.g., carboxylic acid esters; dehydration, e.g., amino acids, and dehalogenation, e.g., organic acid chlorides. Other such polyamide progenitors will be obvious to one skilled in the art.
The polyamide resins of the present invention may be prepared in a single step polymerization or cocondensation of a mixture having essentially equimolar or stoichiometric amounts of organic carboxylic acid functionality and organic amine functionality distributed among the three generic classes or constituent class (a, b, or c above) of compounds from which the instant polyamides derive. It has been found that only the combination of the polyamide-forming short chain component, the polyamide-forming dimer acid component, and the polyamide-forming long-chain oligomer or amorphous component in the mole percent ranges indicated produces the advantageous properties of the present polyamides as hereinafter more particularly described.